We propose an optimal way to use horizontal‐to‐vertical spectral ratios (HVRs) for subsurface structure exploration, based on the diffuse field concept (Kawase et al., 2011; Sanchez‐Sesma et al., 2011). This approach is applicable to both earthquake and microtremor ground motions. We show here analyses of the observed ground‐motion data at and around a K‐NET station in Miyagi Prefecture, Japan, where very large peak horizontal ground acceleration was observed during the earthquake of 11 March 2011 off the Pacific coast of Tohoku, Japan. We compare HVRs of the strong motions for the mainshock and the largest peak acceleration aftershock with those averaged over tens of weak motions to observe soil nonlinearity effects on the HVRs. Then, we determine detailed velocity profiles from the HVRs at the K‐NET Tsukidate station and the temporary aftershock observation sites. We find that HVRs can be explained quite well by the identified velocity profiles at all the target sites. The observed peak at 9 Hz for the averaged weak‐motion data originates in the topmost layers lying over the engineering bedrock.